1. Field of the Invention
The present invention relates in general to marine riser tensioners and, in particular, to a RAM style push up tensioner that accommodates riser tilt.
2. Brief Description of Related Art
Offshore production platforms must support production risers from oil or gas wells that extend to the platform from subsea wells. For platforms that are fixed to the ocean floor this is readily accomplished and is well known in the art. However, for subsea completions in deep water that require the use of floating platforms, such as tension leg platforms (TLPs) or semi-submersible platforms, supporting risers presents significant problems. These platforms move under the influence of waves, wind, and current and are subjected to various forces. Thus, the riser tensioning mechanism must permit the platform to move relative to the riser.
The riser tensioning mechanism must also maintain the riser in tension so that the entire weight of the riser is not transferred to the wellhead and so that the riser does not collapse under its own weight. The tensioning mechanism must therefore exert a continuous tensional force on the riser. Also, this force must be maintained within a narrow tolerance.
Push up tensioners have several advantages in subsea applications, one being that the tensioner accommodates higher loads in a smaller space over other types of tensioners. This is in part because push up tensioners use a more efficient piston end and do not require a tension pulling device at the end connection. In addition, the pressure in push up tensioners does not act on the rod side of the cylinder. Where seas are rough, and the floating platform experiences great range of vertical motion, push up tensioners are better able to accommodate that vertical motion. In addition, use of a push-up tensioner can minimize the corrosive effects of the salt-water environment in which they must operate because the high pressure seals of the tensioner are not located adjacent to the atmosphere and are isolated from caustic fluids and debris.
TLPs provide stable drilling platforms in deeper waters. In TLPs, tension legs extends from the platform down to an anchor located at the sea floor. The tension legs are relatively inelastic meaning that much of the vertical motion of the platform is eliminated. TLPs allow for location of the wellhead assembly on the surface rather than on the sea floor. A riser will typically extend from the wellhead assembly down to the sea floor. This setup allows for simpler well completion and better control of production. However, in TLPs the riser may tilt from the vertical relative to the TLP. The amount of riser tilt from the vertical is not static and varies with time during operation.
While use of both TLPs and RAM type push up tensioners is desired, because of the varying riser tilt, RAM style push-up tensioners constructed to date are not currently suitable for use with TLPs. In all previous RAM systems, the cylinders remain in line with the riser, which allows for small spacing of the risers. While the small size of the RAM style push up tensioner is desirable, the small size also causes a problem in that it limits the size of the passage in which the riser may be run. Therefore, there is a need for a push up riser tensioner that can tilt with the riser and allow suitable space for running of a riser for use in a TLP.